As a power supply for driving modern mobile electronic instruments such as a mobile phone, mobile personal computer, or mobile music player and further as a power supply for electric tools, hybrid electric vehicles (HEVs), or electric vehicles (EV5), a nonaqueous electrolyte secondary battery represented by a lithium-ion secondary battery having high energy density and with high capacity is extensively utilized for low-load and high-load applications.
As a positive electrode active material of these nonaqueous electrolyte secondary batteries, lithium transition metal complex oxides represented by LiMO2 (where M is at least one of Co, Ni, and Mn) capable of reversibly absorbing and desorbing lithium ions, i.e., a single one of or a mixture of a plurality of LiCoO2, LiNiO2, LiNiyCo1-yO2 (y equals 0.01 to 0.99), LiMnO2, LiCoxMnyNizO2 (x+y+z=1), LiMn2O4, LiFePO4, or the like is used. Of these, lithium-cobalt complex oxides or lithium-cobalt complex oxide added with a dissimilar metal element is frequently used due to superiority over others in various battery characteristics.
However, since cobalt is expensive and the existing amount as a resource is small, many attempts have been made until now to achieve various battery characteristics comparable or superior to a case where cobalt is used by using other transition metals instead of cobalt. Of these, lithium-containing nickel oxide active material is expected to be a promising material in that the theoretical capacity is large and the charge-discharge potential is high, and so is lithium-containing manganese oxide active material in that the cost is low and the thermal stability is excellent. However, lithium-containing nickel oxide active material has a problem that the thermal stability is low, and lithium-containing manganese oxide active material has a problem in that the theoretical capacity is small and the cycle life at high temperature is short.
Although mixing spinel lithium manganese oxide with nickel oxide active material tends to improve safety performance, the addition of lithium manganese oxide in large amounts reduces the capacity and causes a decrease in high-temperature storage and high-rate charge-discharge cycle performance. As described above, it has been difficult with conventional methods to realize both an improvement in safety performance as well as in favorable (high-load) cycle performance and an increase in capacity.
For the problem described above with each of these lithium-containing nickel oxide active material and lithium-containing manganese oxide active material, Patent Document 1, for example, discloses a technique in which improvements in thermal stability and discharge capacity are both realized by mixing, as a positive electrode material, fluorine-added lithium-nickel-cobalt-manganese complex oxide represented by LiNi1−x−yCoxMnyO2 (x and y satisfy the condition of 0.5<x+y<1.0 and 0.1<y<0.6 in the formula) and lithium-manganese complex oxide having a spinel structure represented by Li(1+a)Mn2−a−bMbO4 (M is at least one element selected from the group consisting of Al, Co, Ni, Mg, and Fe, and conditions of 0≦a≦0.2 and 0≦b≦0.1 are satisfied in the formula).
Patent Document 2 discloses that, in a nonaqueous electrolyte secondary battery in which a coating is formed on the surface of a carbon material as a negative active material by causing a particular cyclic carbonate ester to be contained within a nonaqueous electrolyte so as to improve the charge-discharge cycling characteristics, a nonaqueous electrolyte secondary battery with improved output characteristics and charge-discharge cycle life can be obtained by combining lithium-manganese complex oxide and lithium-nickel-cobalt-manganese complex oxide as a positive electrode active material, the lithium-manganese complex oxide represented by composition formula LixMn2−y1M1y2O4+z (M1 is at least one element selected from the group consisting of Al, Co, Ni, Mg, and Fe, and conditions of 0≦x≦1.5, 0≦y1≦1.0, 0≦y2≦0.5, and −0.2≦z≦0.2 are satisfied in the formula) having a spinel structure, and the lithium-nickel-cobalt-manganese complex oxide represented by composition formula LiaNibCocMndO2 (where conditions of 0≦a≦1.2 and b+c+d=1 are satisfied).    Patent Document 1: JP-A-2005-267956    Patent Document 2: JP-A-2004-146363    Patent Document 3: JP-A-2006-278322